Janine K. Kruit

β-cell ABCA1 influences insulin secretion, glucose homeostasis and response to thiazolidinedione treatment

Type 2 diabetes is characterized by both peripheral insulin resistance and reduced insulin secretion by β-cells. The reasons for β-cell dysfunction in this disease are incompletely understood but may include the accumulation of toxic lipids within this cell type. We examined the role of Abca1, a cellular cholesterol transporter, in cholesterol homeostasis and insulin secretion in β-cells. Mice with specific inactivation of Abca1 in β-cells had markedly impaired glucose tolerance and defective insulin secretion but normal insulin sensitivity. Islets isolated from these mice showed altered cholesterol homeostasis and impaired insulin secretion in vitro. We found that rosiglitazone, an activator of the peroxisome proliferator–activated receptor-γ, which upregulates Abca1 in β-cells, requires β-cell Abca1 for its beneficial effects on glucose tolerance. These experiments establish a new role for Abca1 in β-cell cholesterol homeostasis and insulin secretion, and suggest that cholesterol accumulation may contribute to β-cell dysfunction in type 2 diabetes.

Reduced cholesterol absorption upon PPARdelta activation coincides with decreased intestinal expression of NPC1L1.

Peroxisome proliferator-activated receptors (PPARs) control the transcription of genes involved in lipid metabolism. Activation of PPARδ may have antiatherogenic effects through the increase of plasma HDL, theoretically promoting reverse cholesterol transport from peripheral tissues toward the liver for removal via bile and feces. Effects of PPARδ activation by GW610742 were evaluated in wild-type and Abca1-deficient (Abca1−/−) mice that lack HDL. Treatment with GW610742 resulted in an ∼50% increase of plasma HDL-cholesterol in wild-type mice, whereas plasma cholesterol levels remained extremely low in Abca1−/− mice. Yet, biliary cholesterol secretion rates were similar in untreated wild-type and Abca1−/− mice and unaltered upon treatment. Unexpectedly, PPARδ activation led to enhanced fecal neutral sterol loss in both groups without any changes in intestinal Abca1, Abcg5, Abcg8, and 3-hydroxy-3-methylglutaryl-coenzyme A reductase expression. Moreover, GW610742 treatment resulted in a 43% reduction of fractional cholesterol absorption in wild-type mice, coinciding with a significantly reduced expression of the cholesterol absorption protein Niemann-Pick C1-like 1 (Npc1l1) in the intestine. PPARδ activation is associated with increased plasma HDL and reduced intestinal cholesterol absorption efficiency that may be related to decreased intestinal Npc1l1 expression. Thus, PPARδ is a promising target for drugs aimed to treat or prevent atherosclerosis.

Macrophage ABCG1 Deletion Disrupts Lipid Homeostasis in Alveolar Macrophages and Moderately Influences Atherosclerotic Lesion Development in LDL Receptor-Deficient Mice

Objective—ABCG1 has recently been identified as a facilitator of cellular cholesterol and phospholipid efflux to high-density lipoprotein (HDL). Its expression in macrophages is induced during cholesterol uptake in macrophages and by liver X receptor (LXR). The role of macrophage ABCG1 in atherosclerotic lesion development is, however, still unknown. Methods and Results—To assess the role of macrophage ABCG1 in atherosclerosis, we generated low-density lipoprotein (LDL) receptor knockout (LDLr−/−) mice that are selectively deficient in macrophage ABCG1 by using bone marrow transfer (ABCG1−/− → LDLr−/−). Peritoneal macrophages isolated from donor ABCG1−/− mice exhibited a 22% (P=0.0007) decrease in cholesterol efflux to HDL. To induce atherosclerosis, transplanted mice were fed a high-cholesterol diet containing 0.25% cholesterol and 15% fat for 6 and 12 weeks. Serum lipid levels and lipoprotein profiles did not differ significantly between ABCG1−/− → LDLr−/− mice and controls. In lungs of ABCG1−/− → LDLr−/− mice a striking accumulation of lipids was observed in macrophages localized to the subpleural region. After 6 weeks of high-cholesterol diet feeding the atherosclerotic lesion size was 49±12×103 &mgr;m2 for ABCG1+/+ → LDLr−/− mice versus 65±15×103 &mgr;m2 for ABCG1−/− → LDLr−/− mice and after 12 weeks of high-cholesterol diet feeding 124±17×103 &mgr;m2 for ABCG1+/+ → LDLr−/− mice versus 168±17×103 &mgr;m2 for ABCG1−/− → LDLr−/− mice. Atherosclerotic lesion size depended on both time and the macrophage ABCG1 genotype (P=0.038 by 2-way ANOVA, n≥8), indicating a moderately 33% to 36% increase in lesion formation in the absence of macrophage ABCG1. Conclusions—Macrophage ABCG1 deficiency does lead to heavy lipid accumulation in macrophages of the lung, and also a moderately significant effect on atherosclerotic lesion development was observed.

Tissue-Specific Induction of Intestinal ABCA1 Expression With a Liver X Receptor Agonist Raises Plasma HDL Cholesterol Levels

ABCA1 controls the rate-limiting step in HDL particle formation and is therefore an attractive molecular target for raising HDL levels and protecting against atherosclerosis. Intestinal ABCA1 significantly and independently contributes to plasma HDL cholesterol levels in mice, suggesting that induction of intestinal ABCA1 expression may raise plasma HDL cholesterol levels. We evaluated the ability of a synthetic Liver X Receptor (LXR) agonist, GW3965, to raise plasma HDL cholesterol levels in control mice and mice with liver- or intestinal-specific deletion of the Abca1 gene. Oral treatment with GW3965 increased the expression of ABCA1 by ≈6-fold (P=0.004) as well as other LXR target genes in the intestines of mice, with no change in the hepatic expression of these genes. This resulted in a significant ≈48% elevation of plasma HDL cholesterol levels in wild-type mice (P<0.01) with no change in plasma triglycerides. A similar increase in HDL cholesterol was observed in mice lacking hepatic ABCA1, indicating that the increase in plasma HDL cholesterol was independent of hepatic ABCA1. This effect was completely abrogated in mice lacking intestinal ABCA1. These data indicate that intestinal ABCA1 may be an attractive therapeutic target for raising HDL levels while avoiding the hepatic lipogenesis and hypertriglyceridemia typical of systemic LXR activation.

Specific Loss of Brain ABCA1 Increases Brain Cholesterol Uptake and Influences Neuronal Structure and Function

The expression of the cholesterol transporter ATP-binding cassette transporter A1 (ABCA1) in the brain and its role in the lipidation of apolipoproteins indicate that ABCA1 may play a critical role in brain cholesterol metabolism. To investigate the role of ABCA1 in brain cholesterol homeostasis and trafficking, we characterized mice that specifically lacked ABCA1 in the CNS, generated using the Cre/loxP recombination system. These mice showed reduced plasma high-density lipoprotein (HDL) cholesterol levels associated with decreased brain cholesterol content and enhanced brain uptake of esterified cholesterol from plasma HDL. Increased levels of HDL receptor SR-BI in brain capillaries and apolipoprotein A-I in brain and CSF of mutant mice were evident. Cholesterol homeostasis changes were mirrored by disturbances in motor activity and sensorimotor function. Changes in synaptic ultrastructure including reduced synapse and synaptic vesicle numbers were observed. These data show that ABCA1 is a key regulator of brain cholesterol metabolism and that disturbances in cholesterol transport in the CNS are associated with structural and functional deficits in neurons. Moreover, our findings also demonstrate that specific changes in brain cholesterol metabolism can lead to alterations in cholesterol uptake from plasma to brain.

Carriers of Loss-of-Function Mutations in ABCA1 Display Pancreatic Beta Cell Dysfunction

OBJECTIVE Abnormal cellular cholesterol handling in islets may contribute to β-cell dysfunction in type 2 diabetes. β-Cell deficiency for the ATP binding cassette transporter A1 (ABCA1), which mediates the efflux of cellular cholesterol, leads to altered intracellular cholesterol homeostasis and impaired insulin secretion in mice. We aimed to assess the impact of ABCA1 dysfunction on glucose homeostasis in humans. RESEARCH DESIGN AND METHODS In heterozygous carriers of disruptive mutations in ABCA1 and family-based noncarriers of similar age, sex, and BMI, we performed oral glucose tolerance tests (OGTTs) (n = 15 vs. 14) and hyperglycemic clamps (n = 8 vs. 8). RESULTS HDL cholesterol levels in carriers were less than half those in noncarriers, but LDL cholesterol levels did not differ. Although fasting plasma glucose was similar between groups, glucose curves after an OGTT were mildly higher in carriers than in noncarriers. During hyperglycemic clamps, carriers demonstrated lower first-phase insulin secretion than noncarriers but no difference in insulin sensitivity. The disposition index (a measure of β-cell function adjusted for insulin sensitivity) of the carriers was significantly reduced in ABCA1 heterozygotes. CONCLUSIONS Carriers of loss-of-function mutations in ABCA1 show impaired insulin secretion without insulin resistance. Our data provide evidence that ABCA1 is important for normal β-cell function in humans.

Cholesterol in islet dysfunction and type 2 diabetes

Type 2 diabetes (T2D) frequently occurs in the context of abnormalities of plasma lipoproteins. However, a role for elevated levels of plasma cholesterol in the pathogenesis of this disease is not well established. Recent evidence suggests that alterations of plasma and islet cholesterol levels may contribute to islet dysfunction and loss of insulin secretion. A number of genes involved in lipid metabolism have been implicated in T2D. Recently an important role for ABCA1, a cellular cholesterol transporter, has emerged in regulating cholesterol homeostasis and insulin secretion in pancreatic beta cells. Here we review the impact of cholesterol metabolism on islet function and its potential relationship to T2D.

HDL and LDL cholesterol significantly influence β-cell function in type 2 diabetes mellitus

Purpose of review Patients with type 2 diabetes mellitus (T2DM) display significant abnormalities in both LDL and HDL particles. Recent data suggest that these changes in lipoprotein particles could contribute to the pathogenesis of T2DM. In this review, we focus on these abnormalities and discuss their possible impact on β-cell function and β-cell mass. Recent findings Infusion of reconstituted HDL in T2DM patients improves β-cell function, whereas carriers of loss-of-function mutations in the cholesterol transporter ABCA1, who have decreased HDL levels, have impaired β-cell function. In addition, recent studies show that HDL protects against stress-induced β-cell apoptosis in vitro. Finally, increasing evidence points to a role for islet inflammation in the pathogenesis of T2DM. ABCA1 and ABCG1 may also modulate these inflammatory responses, suggesting an additional pathway by which HDL may impact T2DM. Summary Recent findings indicate that HDL protects β-cells from cholesterol-induced β-cell dysfunction, stress-induced apoptosis and islet inflammation. As the protective properties of HDL are compromised in patients with metabolic syndrome and T2DM, dysfunctional HDL metabolism could contribute to the pathogenesis of T2DM. Therapeutic normalization of both the quantity and quality of HDL particles may be a novel approach to prevent or treat T2DM.

A functional ABCA1 gene variant is associated with low HDL-cholesterol levels and shows evidence of positive selection in Native Americans

It has been suggested that the higher susceptibility of Hispanics to metabolic disease is related to their Native American heritage. A frequent cholesterol transporter ABCA1 (ATP-binding cassette transporter A1) gene variant (R230C, rs9282541) apparently exclusive to Native American individuals was associated with low high-density lipoprotein cholesterol (HDL-C) levels, obesity and type 2 diabetes in Mexican Mestizos. We performed a more extensive analysis of this variant in 4405 Native Americans and 863 individuals from other ethnic groups to investigate genetic evidence of positive selection, to assess its functional effect in vitro and to explore associations with HDL-C levels and other metabolic traits. The C230 allele was found in 29 of 36 Native American groups, but not in European, Asian or African individuals. C230 was observed on a single haplotype, and C230-bearing chromosomes showed longer relative haplotype extension compared with other haplotypes in the Americas. Additionally, single-nucleotide polymorphism data from the Human Genome Diversity Panel Native American populations were enriched in significant integrated haplotype score values in the region upstream of the ABCA1 gene. Cells expressing the C230 allele showed a 27% cholesterol efflux reduction (P< 0.001), confirming this variant has a functional effect in vitro. Moreover, the C230 allele was associated with lower HDL-C levels (P = 1.77 x 10(-11)) and with higher body mass index (P = 0.0001) in the combined analysis of Native American populations. This is the first report of a common functional variant exclusive to Native American and descent populations, which is a major determinant of HDL-C levels and may have contributed to the adaptive evolution of Native American populations.

Cholesterol efflux via ATP-binding cassette transporter A1 (ABCA1) and cholesterol uptake via the LDL receptor influences cholesterol-induced impairment of beta cell function in mice

Aims/hypothesisCellular cholesterol accumulation is an emerging mechanism for beta cell dysfunction in type 2 diabetes. Absence of the cholesterol transporter ATP-binding cassette transporter A1 (ABCA1) results in increased islet cholesterol and impaired insulin secretion, indicating that impaired cholesterol efflux leads to beta cell dysfunction. In this study, we aimed to determine the role of the LDL receptor (LDLr) in islet cholesterol uptake and to assess the contributions of cholesterol uptake compared with efflux to islet cholesterol levels.MethodsIslet cholesterol and beta cell function were assessed in mice lacking LDLr (Ldlr−/−), or apolipoprotein E (Apoe−/−), as well as in mice with beta-cell-specific deficiency of Abca1 crossed to Ldlr−/− mice.ResultsHypercholesterolaemia resulted in increased islet cholesterol levels and decreased beta cell function in Apoe−/− mice but not in Ldlr−/− mice, suggesting that the LDL receptor is required for cholesterol uptake leading to cholesterol-induced beta cell dysfunction. Interestingly, when wild-type islets with functional LDL receptors were transplanted into diabetic, hypercholesterolaemic mice, islet graft function was normal compared with Ldlr−/− islets, suggesting that compensatory mechanisms can maintain islet cholesterol homeostasis in a hypercholesterolaemic environment. Indeed, transplanted wild-type islets had increased Abca1 expression. However, lack of the Ldlr did not protect Abca1−/− mice from islet cholesterol accumulation, suggesting that cholesterol efflux is the critical regulator of cholesterol levels in islets.Conclusions/interpretationOur data indicate that islet cholesterol levels and beta cell function are strongly influenced by LDLr-mediated uptake of cholesterol into beta cells. Cholesterol efflux mediated by ABCA1, however, can compensate in hypercholesterolaemia to regulate islet cholesterol levels in vivo.